The present invention generally pertains to detecting and processing signals from received high frequency electromagnetic radiation signals, and is particularly directed to maintaining a normalized phase relationship in the respective channels of a system having a plurality of tuned receiver channels.
The technology of detecting and processing intelligible signals from propogated electromagnetic energy has, since the early development of radio, almost totally utilized a down converter system. Basically, the down converter system converts the higher carrier signal frequencies to a lower frequency that is more practical to filter, amplify and process. The conversion involves the technique of modulating the received signal frequency f1 with a controlled local oscillator signal frequency f2 then selecting a specific product frequency from the spectral components generated by the inter-modulation process. This process may be expressed by the following relationship.
M(f1).+-.N(f2)=fn where: PA1 M and N are integers defining the harmonic multipliers; PA1 f1 is the frequency of a received signal; PA1 f2 is the frequency of the local oscillator frequency; and PA1 fn is the frequency produced by the sum or difference of f1 and f2 at any one of the various harmonics.
It is a general practice to select the lowest product (f1-f2) or (f2-f1) and suppress the others by a post-selective bandpass filter. Unfortunately the desired fn (or an intermediate frequency) can be reproduced by many harmonic combinations of f1 and f2. In most applications, this disadvantage of the down-converter system can be sufficiently overcome with a pre-selection circuit in the form of a bandpass filter tuned for the desired signal frequency.
In certain missle guidance systems, a configuration of parallel receiver channels for receiving and amplifying pulsed high frequency carrier signals is required. These receiver channels may be required to be tunable across a relatively wide frequency range with a minimal phase shift between the channels. This requirement precludes the use of tunable preselection filters which are inherently poor in phase tracking, particularly across a wide frequency range. Without preselective filtering, a down converter system is responsive to the various combinations of spurious frequencies f1 and harmonics thereof that will intermodulate with the various harmonics of f2 and produce the proper intermediate frequency.
A recognized alternative to the down-converter system is a system including tuned frequency amplifiers. Tuned frequency amplifiers provide an amplified signal that is effectively free of intermodulation products. However, the tuned frequency amplifier is highly unpredictable in phasing when tuned across a wide frequency band. Therefore, it has not been used in guidance systems where parallel channels having a normalized phase relationship are required.
The object of the present invention is to compensate for the phase tracking deficiency common to parallel channel tuned frequency amplifier systems.